I wanted to tag my music library with the BPM of each song. There are commercial products for doing this through iTunes, but I wanted to do it from the command line, because I also use my music library outside of iTunes, and I don't want to be tied to iTunes anyway.

The bpm-tools package can calculate bpm for a song. On Mac, it's easy to install with Homebrew. The base package only includes the analyzer; I wanted something to tag my MP3 files too, so I used the =-with-bpm-tag= option. It ends up installing dependencies: mad, sox, id3lib, id3v2, flac, libogg, libvorbis, libao, vorbis-tools.